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Introduction

Phosphate functional monomers are designed to chemically interact with dentinal hydroxyapatite, being thought of importance for bond durability. Among many functional monomers, 10-MDP was most shown to improve bonding effectiveness in laboratory and clinical research . In our previous research, 10-MDP’s chemical interaction with hydroxyapatite (HAp) of tooth tissue was proven using diverse chemical analytical tools, such as XRD, XPS and NMR . Furthermore, 10-MDP was documented to self-assemble into so-called ‘nano-layering’, a process driven by the deposition of 10-MDP_Ca salts with low solubility . Each nano-layering consists of two sublayers of parallel oriented 10-MDP monomers in opposite direction. 10-MDP’s methacrylate group is directed inwards, enabling mutual co-polymerization between two opposed monomers. Its functional phosphate group is directed outwards, capturing Ca released from dentin thanks to the etching effect of 10-MDP and so coupling adjacent nano-layers. Further in-depth research into the process of nano-layering is warranted to assess the relevance and actual contribution of this compact 3D structure to the stability of the adhesive interface.

The functional monomer 10-MDP was originally synthesized by Kuraray (today Kuraray Noritake, Tokyo, Japan) and has been introduced in 1981 (following ‘Clearfil SE Bond’ Technical Information from Kuraray Noritake). The actual 10-MDP patent expired in 2011, and has led other companies to develop and launch 10-MDP adhesives, many of them have been referred to as so-called ‘universal’ adhesives, such as Adhese Universal (Ivoclar Vivadent, Schaan, Liechtenstein), All-Bond Universal (Bisco, Schaumburg, IL, USA), Clearfil Universal Bond (Kuraray Noritake), Futurabond U (Voco, Cuxhaven, Germany; although the MSDS document does not specify what the ‘acidic adhesive monomer’ is), G-Premio Bond (GC, Tokyo, Japan) and Scotchbond Universal (3M ESPE, Seefeld, Germany). Besides indicated for direct and indirect restorative procedures, universal adhesives allow the dentist to opt for either an etch-and-rinse (E&R) or self-etch (SE) application protocol.

Bonding effectiveness of the different commercial 10-MDP-based adhesives was found to vary . Self-evidently, difference in composition is the most plausible reason. Bond strength was shown to depend on the actual concentration of 10-MDP . The interfacial interaction potential of 10-MDP with HAp was found to be inhibited by other monomers like HEMA . 10-MDP’s performance may also degrade with time, as the monomer is sensitive to hydrolytic degradation .

To date, the effect of 10-MDP’s purity on bonding effectiveness is unknown, although several chemical companies (PCM, Krefeld, Germany; DMI, San Diego, CA) commercially offer the functional monomer to be used for dental purposes. Different purities are reported in the accompanied technical documentation. Moreover, patent literature not clearly describes the synthesis process and actual purification process . We therefore investigated in this study three different 10-MDP versions, provided by three different companies, on their purity and their chemical interaction potential as well as bonding effectiveness to dentin. The null hypothesis tested was that the adhesive performance of experimental primers varying for the 10-MDP version was not different when they were used as part of a 2-step SE adhesive protocol.